Recent discovery of T-helper 17 (Th17) cells and regulatory T cells (Treg) has markedly facilitated our understanding of human immunity under both physiological and pathological conditions. Tregs have a broad immunosuppressive capacity and play a central role in the prevention of autoimmunity and maintenance of immune homeostasis, whereas Th17 cells play an important role in host defense against microbial infections, and are important contributors to the pathogenesis of a wide array of inflammatory and autoimmune diseases. However, Treg cells also have deleterious effects by aiding the persistence of infectious pathogens and blocking effective anti-tumor immunity. A better understanding of the nature and intimate links between Th17 and Treg cell responses will be critical for the elucidation of the roles of these cells in the pathogenesis of different diseases, and for the development of novel strategies to treat and prevent immune-related diseases including cancer and autoimmune diseases. We recently demonstrated that increased Th17 cell populations exist in the tumor-infiltrating T cells (TILs) of melanoma, ovarian, breast and colon cancers. We further discovered that human tumor-infiltrating Th17 cells can differentiate into IFN-?-producing and FoxP3+ populations possessing potent suppressive activity after multiple in vitro T cell receptor (TCR) stimulations and expansions, suggesting the instability and plasticity of tumor-infiltrating Th17 cells. Therefore, it is critical to investigate the commitment, plasticity and interconversion between Th17 cells and Tregs, and understand the molecular mechanisms responsible for the Th17-to-Treg differentiation in the tumor microenvironment. Our long-term goals are to identify the role of Th17 and Tregs in anti-tumor immunity and develop strategies to modify their interactions and functions for improved cancer treatment. The central hypothesis of this proposal is that Th17 cells have developmental plasticity and can differentiate into Tregs with suppressive function in the tumor microenvironment. Specific Aim 1 seeks to identify the generality of differentiation of Th17 cells derived from non-tumor sites into Tregs, and then to identify the potential molecular mechanisms, including epigenetic modification of FoxP3 expression and reprogramming of lineage-specific transcriptional factor and cytokine genes, involved in the Th17-to-Treg differentiation and conversion. Specific Aim 2 will investigate the differentiation of Th17 cells into Tregs in vivo in mouse tumor models. We will first investigate whether tumor microenvironments promote the conversion of Th17 cells into Tregs in a B16 melanoma tumor model using highly purified Th17 cells polarized from FoxP3-CD4+ T cells derived from FoxP3EGFP transgenic mice. We will then determine whether antigen stimulation is critical for the differentiation of Th17 cells into Tregs in the tumor microenvironment using OT-II and FoxP3EGFP double transgenic mice. A positive outcome of these studies should lead to novel strategies for manipulation of Th17 and Treg cell commitment for the treatment of human cancer and other diseases as well.